An inkjet printing apparatus capable of a color output generally comprises a plurality of printheads for discharging inks of four, yellow, cyan, magenta, and black colors (to be referred to as Y, M, C, and K hereinafter). Recently, to suppress dot graininess at highlights, many inkjet printing apparatuses adopt an arrangement of forming a color image using inks of six or more colors including light inks prepared by decreasing the densities of respective colors (e.g., light cyan and magenta inks prepared by decreasing the densities of cyan and magenta inks: to be referred to as LC and LM hereinafter), and inks having high concentrations that are usually used.
In general, four, Y, M, C, and K color inks or six, Y, M, C, K, LC, and LM color inks print data with their dedicated printheads. The discharge amount of each printhead varies owing to variations in structure during the printhead manufacturing process. For example, each discharge amount vary about ±10% with respect to a standard discharge amount. The variations in the discharge amount of each printhead result in different discharge amounts of the respective colors. As a result, the image density and tint vary.
The color tone of an output image is determined in accordance with the standard discharge amount of the printhead in terms of the printer design. An image printed by a printer using a printhead whose discharge amount is deviated from the standard one exhibits different color tone. As the image quality of recent inkjet printers increases, images almost equivalent to silver halide photographic images can be obtained. For the photographic image, the color tone is an important factor for determining the image quality. A color tone different from the designed value causes    (1) low reproducibility    (2) omission of tone levels (in particular, low tone level reproducibility owing to unbalanced dark and light inks of the same color, failure to obtain linear tonability, and the like)    (3) generation of a pseudo outlineThis greatly degrades the image quality.
To solve this problem, there is a conventional method of printing a test pattern for determining variations in discharge amount, scanning the printed test pattern with a scanner to determine the discharge amount, and changing the processing parameters of image processing. This method can avoid degradation in image quality. However, the user must output a test pattern, a reading device such as a scanner is required, and the system becomes complicated and expensive.
There is another method of outputting a test pattern for determining the discharge amount, visually checking the test pattern by the user, and inputting the result by the user via the user interface of a host computer to correct the color tone. However, this method depends on a visual check by the user, so erroneous determination or an input error may occur. This may further degrade an image.
Japanese Patent Laid-Open No. 6-320732 discloses a technique of storing information about a printhead in an EEPROM attached to the printhead, and performing optimal head driving control based on the contents to increase the image quality. This method can stabilize a discharge state, but may fail to correct variations in discharge amount. Inkjet printheads disclosed in Japanese Patent Laid-Open Nos. 54-161935, 61-185455, 61-249768, and 4-10941 are characterized by a stable discharge amount regardless of head driving conditions and the like. However, correction under head driving control cannot increase the image quality.